I just recently had my washing machine overflow from the standpipe. I have never had an issue with this until the other day. It overflows initially when the washing machine begins to drain. Once the intital draining slows down it does not overflow anymore. The standpipe is approx. 30" high and several inches abover the washing machine. I believe there may be a clog since the problem just started. Any help would be appreciated.

As you correctly guessed, there is a clog inside the pipe. First, I would make sure the P-trap is clean. If so, snaked it with 3/8" snake. Run the snake its full length.

There should be Clean Out plug somewhere in the vicinity of the stand pipe. If not than you will have to remove P-trap to get to the drain. Reinstall P-trap afterward...

Is there any other pluming fixtures connected to this drain pipe ? Like kitchen sink ?

Another way to snake the clog would be to locate the washer roof vent and snake from there, Put out enough snake to reach the base and 20 feet more. Good luck, Tom